A limited though persuasive body of evidence indicates that many physicians narrow their scope of practice to a small set of services chosen from the larger set of services for which they are trained. This phenomenon of "superspecialization" has important implications for the quality and cost of health care. The objective of this project is to examine the patterns and determinants of superspecialization among medical and surgical subspecialists. The specific aims are: 1. To describe the patterns of superspecialization among medical and surgical subspecialists, and to assess whether these patterns are consistent over time. 2. To examine the determinants of overall degree of superspecialization (i.e., narrowness of practice scope) among subspecialists, including physician and market factors. 3. To examine the determinants of the extent to which subspecialists focus their practices on surgical procedures, consultations, and primary care. The conceptual framework for the empirical analyses is based on the economic theory of specialization as applied to the physician services market. Three medical and four surgical subspecialties will be studied. Sources of data will include 1992 and 1993 Medicare physician eligibility and claims files, in addition to other secondary data sources. Degree of superspecialization will be measured using an information theory index. Analyses will divided into three tasks corresponding to the three specific aims of the project. Task 1 will describe patterns of superspecialization and examine variations in practice scope among subspecialists. Task 2 will use linear regression models to assess the effects on degree of superspecialization of variables such as subspecialist and primary care physician supply, HMO market penetration, and group practice. Task 3 will use linear models to examine the effects of these variables on the extent to which subspecialists concentrate their practices on surgical procedures, consultations, and primary care. The findings of this project will enable us to anticipate how proposed physician workforce policies and the evolution of health care delivery toward managed care are likely to influence subspecialists' scope of practice. This information should prove quite useful in policy debates about how best to reform our health care system and restructure our physician workforce. The project also will make a major contribution to the literature on the determinants of physician practice patterns.